This invention relates to a diametrical expansion treatment of polytetrafluoroethylene (ptfe) resin tubes, and more particularly to a method of manufacturing so-called contraction tubes having a memory function in which the tubes maintain their shapes at room temperature, but when heated above a certain threshold temperature their shapes are changed and returned back to their original shapes.
A manufacturing method in which, after being irradiated and suitably bridged, a mixture of theroplastic material and polyethylene, polyvinylchloride or neoprenesilicon is subjected to expansion treatment, to thereby function as a contraction tube, is taught by U.S. Pat. Nos. 3,086,242 and 3,370,112. Since an extruded pfte resin tube has a considerably high melting viscosity at a temperature above 327.degree. C, by cooling it below 327.degree. C under pressure a tube having the same contractivity can be obtained. However, in the extruded ptfe resin, crystalline particles are arranged in the extrusion direction but not in the lateral direction, as is well known in the art. This orientation characteristic remains even after heat treatment at a temperature higher than 327.degree. C. Therefore, when in order to radially expand the extruded tube, the outer surface of the tube is subjected to a negative or vacuum pressure and high pressure air is introduced into the inside of the tube, the tube frequently cracks in the longitudinal direction, and it is therefore difficult to manufacture long ptfe resin tubes.
This difficulty can be overcome to some extent by heat-treating the extruded tube without any pressure differential between the inner and outer surfaces thereof, and by radially expanding it at a temperature higher than 327.degree. C with a positive pressure differential between the inner and outer surfaces. This method is not practical in a manufacturing operation, however. Further, if the tube contains a material which allows the contraction of the tube to an extent that a residual stress remains, when the tube is heated cracks are generated in the axial direction of the tube. The frequency of occurrence of and magnitude of the cracks depends on the magnitude of the residual stress. For instance, when the tube is completely contracted to its original diameter, the residual stress becomes zero, and accordingly no crack occurs.
However, in the use of a thermally contractive tube, some residual stress frequently remains. If such residual stress is completely eliminated, the adhesivity between the covering tube and the core material becomes extremely poor. Thus, the conventional ptfe resin tube has a number of drawbacks.